The present invention generally relates to image superimposing apparatuses, and more particularly to an image superimposing apparatus which superimposes a plurality of images to form one image for display.
In a video conference system, for example, a so-called interframe differential system is used for transmitting the image. According to the interframe differential system, differential data are obtained from differential components between frames of the image and transmitted. The original image is reproduced and displayed from the differential data.
The interframe differential system will be described in conjunction with FIGS. 1A and 1B. As shown in FIG. 1A, the image superimposing apparatus is provided with two frame memories 10 and 11 and an operation circuit 13. The operation circuit 13 adds the image data which is read out from the frame memory 10 which is supplying the image data for display on a display device (not shown) and the differential image data which is received via a terminal 12. The image data which is obtained by the addition and is related to the next frame is written into the frame memory 11. Next, as shown in FIG. 1B, the image data read out from the frame memory 11 is supplied to the display device for display, and the image data which is related to the next frame and is obtained in the operation circuit 13 similarly as described above is written into the frame memory 10.
Conventionally, the video conference system separates the portrait image which is related to a person and includes large motion and the background image which is related to the background such as the conference room and includes only small motion. The two kinds of images are transmitted by taking the interframe differential components with a period dependent on the motion included in the images. FIG. 2 shows an example of a conventional image superimposing apparatus which is used for superimposing the two kinds of images.
In FIG. 2, the image data which is obtained in an operation circuit 13a based on the differential data of the background image received from a terminal 12a and the image data related to the present frame is alternately written into frame memories 10a and 11a. Similarly, the image data which is obtained in an operation circuit 13b based on the differential data of the portrait image received from a terminal 12b and the image data related to the present frame is alternately written into frame memories 10b and 11b.
The image data read out from the frame memories 11a and 10b in synchronism with each other are superimposed in a superimposing circuit 15 and supplied to a display device 16. Hence, an image in which a portrait image 16b is superimposed within a background image 16a is displayed on the display device 16.
According to the conventional image superimposing apparatus, however, it is necessary to provide two frame memories for each image which is to be superimposed on another image. For example, six frame memories are required when superimposing three images. For this reason, there is a problem in that the number of frame memories required increases as the number of images to be superimposed increases.
On the other hand, if the images were to be superimposed on the transmitting side and transmitted, the receiving side would only require two frame memories regardless of the number of images which are superimposed. However, it is impossible in this case to concentrate on the transmission of only the portrait image which includes large motion. In addition, it is impossible in this case to superimpose the images which are received from various locations of a multi-point video conference system into one image for display.